Electropulse method of holes boring and boring machine

ABSTRACT

This invention provides an excavator 1 for crushing a matter to be excavated, existing in an excavating hole in which a discharge liquid is fed, by electric discharge between a plurality of electrodes generated by high-voltage pulses. The excavator comprises a high-voltage pulse generator 2; a plurality of electrodes 17, 18, at least one of which is given a high voltage from the high-voltage pulse generator 2; discharge liquid circulating system 3, 4, 5a, 5b; and optimum condition setting devices 13, 14, 15, 16. Also, this invention provides an excavation method in which at least one of the parameters for excavation efficiency of i) load voltage required for crushing the matter to be excavated; ii) single pulse energy; and iii) quantity of discharge liquid, is optimized for minimization of power consumption required for excavation by using the excavator of this invention.

TECHNICAL FIELD

This invention relates to electropulse method of holes boring and boringmachine. In other words, this invention relates to excavation of a solidinsulating matter, mining of oil and gas and civil engineering andconstruction work.

BACKGROUND ART

Excavation methods and excavators using electric pulses are knownhitherto. For example, optimization for crush of a rock mass and aman-made structure by means of electric pulses is described by Vajor V.F., Siomkin B. V., Adam A. M., "Physics Vol. 4", Tomsk PolytechnicUniversity 1996.

According to this known excavation method, a bore top is placed on arock mass in a discharge liquid. High-voltage pulses are applied toelectrodes at intervals of microsecond to allow electric discharge passthrough the rock mass so as to fracture and crush it. The time requiredfor the rock mass to be fractured is determined by a length between theelectrodes. The drawback to this method is that the interval between theelectrodes is only one parameter for increasing excavating efficiencies.

Another conventional type of excavator comprises a high-voltage pulsegenerator, a bore pipe, and a bore top. The bore pipe includes an outerearth pipe and an inner high-pressure pipe arranged concentrically andhas the bore top at the tip end. The drawback to this excavator is noequipment for setting optimum conditions for excavation.

Known as a still another known excavation method and excavator is"Material crush by means of electric pulses" written by B. V. Siomkin,A. F. Uthof, V. I. Bathes (on pages 7-11, 34-62, 220-224, 11-16 and231-240, Nauka Press 1995).

According to this method of testing, a rock mass to be crushed is dippedinto a liquid. The liquid serves as an insulator in a selected pulserange of high-voltage electric pulses. The electric pulses are appliedto the electrodes placed on the rock mass to allow the electricdischarge to occur in the rock mass dipped in the insulating fluid. Thedrawback of this method is that the optimum condition for the crushingof the rock mass is only effective for the rock mass existing betweenthe two electrodes, which is thus largely different from excavation.

A yet another known excavator comprises a bore top, a bore pipe and ahigh-voltage power supply. This known excavator is provided with a guideat an entrance of a hole to be excavated and a lifting device. Thedischarge liquid in the hole is allowed to cycle to be led to adischarge reservoir. The high-voltage pulses are applied to ahigh-pressure pipe of the bore pipe.

The drawback to this known excavator is that the excavating hole is notso sufficient in structure as to reach the maximum efficiency.

In the light of the above-described drawbacks involved in the prior art,the present invention has been made. It is the object of the presentinvention to provide an excavation method and an excavator capable ofexcavating efficiently with a minimum power consumption.

DISCLOSURE OF THE INVENTION

The present invention provides an excavation method for crushing amatter to be excavated, existing in an excavating hole in which adischarge liquid is fed, by means of electric discharge between aplurality of electrodes generated by high-voltage pulses, wherein atleast one of the following parameters for excavation efficiency is setto be an optimum value for minimization of power consumption requiredfor excavation, before performing the excavation:

i) load voltage required for the crush of the matter to be excavated;

ii) single pulse energy; and

iii) quantity of discharge liquid.

Specifically, for the load voltage required for crushing the matter tobe excavated, possible optimum values of the load voltage required forcrushing the matter to be excavated are estimated by the followingEquation (1), followed by finding an optimum value of the load voltagerequired for crushing the matter to be excavated by varying the loadvoltage continuously or intermittently within a range of load voltagescentered near those estimated load voltage values U1:

    U1=K(1/n-1).sup.0.15 ×U.sub.0 ×L.sup.0.4 [kv]  (1)

where

K: a coefficient, K=1.0-1.5 [1/cm⁰.4 ];

n: the number of electrodes;

L: a length between the electrodes [cm]; and

U₀ : a value obtained by testing, or a voltage [kv] applied when asample of the matter to be excavated existing in the discharge liquid iscrushed via two electrodes having the length of 1 cm therebetween placedon the sample.

For the single pulse energy, possible optimum values of the single pulseenergy are estimated by the following Equation (2), followed by findingan optimum value of the single pulse energy by varying the single pulseenergy continuously or intermittently within a range of single pulseenergies including the estimated optimum values W₀ :

    W.sub.0 >90L.sup.1.6 [J]                                   (2)

For the quantity of discharge liquid, possible optimum values of thequantity of discharge liquid are estimated by the following Equation(3), followed by finding an optimum value of the quantity of dischargeliquid by varying the quantity of discharge liquid continuously orintermittently within a range of quantity of discharge liquid includingthose estimated optimum values Q of the quantity of discharge liquid:

    Q=(0.25-0.5)π×Db.sup.2 /4×f [liter/min.]    (3)

where

Db: a diameter of a bore top [cm]; and

f: the number of pulses per second (frequencies of pulse).

According to the excavation method of the present invention, since atleast one of the parameters for excavation efficiencies, i) load voltagerequired for the crush of the matter to be excavated; ii) single pulseenergy; and iii) quantity of discharge liquid, is set to be an optimumvalue for minimization of power consumption required for excavation,before performing the excavation, the power consumption can be kept at aminimum to make excavation with efficiency.

Further, since possible optimum values for minimization of powerconsumption of the parameters for the excavation efficiencies areestimated by the above Equations (1), (2), and (3) before attempts forexcavation, the number of testing for finding an optimum value can bereduced to a minimum number, to find the optimum value with efficiency.

Also, the present invention provides an excavator 1 for crushing amatter to be excavated, existing in an excavating hole in which adischarge liquid is fed, by means of electric discharge between aplurality of electrodes generated by high-voltage pulses, the excavatorcomprising a high-voltage pulse generator; a plurality of electrodes, atleast one of which is given a high voltage from the high-voltage pulsegenerator; discharge liquid circulating system; and optimum conditionsetting devices.

The optimum setting devices are connecting between the high-voltagepulse generator and the plurality of electrodes, or assembled in thedischarge liquid circulating system, or connected between thehigh-voltage pulse generator and the plurality of electrodes andassembled in the discharge liquid circulating system, so that at leastone of the following parameters for excavation efficiencies is optimizedso that power consumption required for excavation can be minimized:i)load voltage required for the crush of the matter to be excavated;

ii) single pulse energy; and

iii) quantity of discharge liquid.

According to the excavator of the present invention, since at least oneof the parameters for excavation efficiencies, i) load voltage requiredfor the crush of the matter to be excavated; ii) single pulse energy;and iii) quantity of discharge liquid, is optimized for minimization ofpower consumption required for excavation, the power consumption can bekept at a minimum to make excavation with efficiency.

Further, the excavator of the present invention includes a bore pipeconnected to the high-voltage pulse generator at one end portion thereofand to the plurality of electrodes at the other end portion thereof, forconveying the high voltage to the electrodes between the high-voltagepulse generator and the plurality of electrodes. The bore pipe includesa high-voltage pipe and a ground pipe arranged concentrically outside ofthe high-voltage pipe, and an inside of the ground pipe and an outsideof the high-voltage pipe of the bore pipe are plated with a non-magneticand high conductive material.

The plating with such a non-magnetic and high conductive materialcontributes to significant reduction of the phenomenon that with anincrease in depth of the excavating hole, a pulse rise time increasesand a voltage amplitude decreases. By virtue of this, the pulseconditions need not be changed so often, thus enabling the stableoperation of the excavator.

In addition, the excavator of the present invention includes a guide,arranged around the bore pipe, for guiding the bore pipe intounderground. The bore pipe and the guide are connected with each otherthrough a sliding contact point so that the bore pipe can slidevertically within the guide.

The contact point via which the bore pipe and the guide are connectedwith each other prevents a possible breakdown of air gap resulting fromthe electric potential difference between the guide and the bore pipe.

Also, in the excavator of the present invention, a high-voltage pulsepower circuit for generating the high-voltage pulses is a high-voltagepulse power circuit of inductor capacitor type in which an inductorcapacitor and a semi-conductor rectifier are combined.

As compared with a conventional type of excavator in which thesemi-conductor rectifier is not used in the power circuit, this type ofpower circuit has the advantages that the form and weight can be cut inhalf to enable movement of the excavator and, further, the number ofcapacitors and sphere gaps are reduced and also the voltage in thecapacitor can be lowered, thus increasing the life of the high-voltagepulse power circuit.

Further, in the excavator of the present invention, there is provided alifting device having a lifting means for moving the bore pipe up anddown, and a high-voltage input portion for inputting a high voltage tothe bore pipe is arranged at one end portion of the bore pipe inclinedfrom an axis of the bore pipe at a predetermined angle.

This inclined arrangement of the high-voltage input portion canfacilitate a connection of the lifting means of the lifting device, suchas the wire, to the bore pipe, as compared with the embodiment of thehigh-voltage input portion arranged coaxially on the bore pipe. This canfacilitate the setting and lifting of the bore pipe in and from theexcavating hole by means of the lifting device without paying particularattention to a contact between the high-voltage input portion and thewire. Further, this enables the bore pipe to be moved in the excavatinghole by means of the lifting device.

In addition, the excavator of the present invention includes a bore pipeincluding a high-voltage pipe and a ground pipe arranged concentricallyoutside of the high-voltage pipe, for conveying the high voltage to theelectrodes between the high-voltage pulse generator and the plurality ofelectrodes, the bore pipe being connected to the high-voltage pulsegenerator at one end portion thereof and to the plurality of electrodesat the other end portion thereof. Also, an outer surface of ahigh-voltage input portion for inputting the high voltage to the borepipe is coated with a semi-conductive material and is electricallyconnected with the ground pipe.

The high-voltage input portion thus coated with the semi-conductivematerial can withstand more load voltage, as compared with the onecoated with no semi-conductive material.

Also, the excavator of the present invention includes a discharge mudcollecting device fixed to the bore pipe. The discharge mud collectingdevice has discharge liquid feeding passageways which are formed bypipes having a segment section and arranged concentrically with the borepipe; and collecting passageways which are formed by grooves definedbetween the discharge liquid feeding passageways and in which aplurality of elastic valves are arranged along a direction of thedischarge mud being collected.

With this structured discharge mud collecting device, even when thespeed allowing the discharge mud to be collected comes to beinsufficient, the discharge mud is dropped onto the elastic valves andthus the back flow is avoided. Further, since the collecting passagewaysare grooves defined between the discharge liquid feeding passageways,not any pipes intended for collecting use, large fragments dropped onthe elastic valves can easily be crushed or pulverized by a knowntechnique. This can eliminate a possible fear, involved in the knowndischarge mud collecting device having a collecting passageway formed bya pipe or equivalent, that the collecting passageway may be blocked bylarge fragments, to cause damage of the collecting pipe and others,which may in turn cause damage of the discharge mud collecting deviceitself.

Further, the excavator of the present invention includes a bore tophaving a plurality of electrodes, at least one of which is given a highvoltage from the high-voltage pulse generator; and a bore pipe, havingone end portion to which the high-voltage pulse generator is connectedand the other end portion to which the bore top is threadedly fixed, forconveying the high voltage to the electrodes between the high-voltagepulse generator and the bore top. A threaded portion of the bore top isprovided with a horizontally extending aperture having a predeterminedlength and two detents arranged in the horizontally extending aperture.Also, a length between the two detents is rendered shorter than thepredetermined length of the horizontally extending aperture, in order toallow the bore top to rotate around its axis.

The bore top provided with the horizontally extending aperture and thedetents can be allowed to rotate around its axis. By virtue of this, thebore top and its joint portions are prevented from being twisted by ashock wave from excavation and an impact from the discharge mud. Thus,the bore top is kept at its specified position by the bottom, thusproducing an enhanced efficiency of the excavation of the rock mass.

Additionally, in the excavator of the present invention, the bore top isprovided with the electrodes on points of intersection of grid and ismovable in the excavating hole.

With this arrangement of the electrodes being arranged on points ofintersection of grid, the electric discharge occurs on the grid and, asa result, the crush occurs also in part of the rock mass protruded inthe part enclosed in the grid, so that the rock mass is crushed withefficiently. Also, with the arrangement of the bore top being movable,holes having different diameters can be excavated by moving the boretop.

Also, in the excavator of the present invention, the high-voltageelectrodes and other electrodes to which the high voltage is input arebent at their ends so that one can extend toward the other with respectto each other.

The electrodes having this structure are suitable for excavation of amatter to be excavated having a core of a large diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a showing of a construction of an excavator of the presentinvention;

FIG. 2 is a showing of a power circuit of the excavator of the presentinvention;

FIG. 3 is a showing of a mounting position of a high-voltage inputportion of the excavator of the present invention;

FIG. 4 is an illustration of the structure of the high-voltage inputportion of the excavator of the present invention;

FIG. 5(a) and FIG. 5(b) are illustrations of a discharge mud collectingdevice forming a discharge liquid circulating system;

FIG. 6 is a schematic diagram of a mounting structure of the bore top tothe bore pipe;

FIG. 7 is a showing of an electrode structure of the bore top;

FIG. 8 is a showing of a structure of tip ends of the electrodes;

FIG. 9 is a graph plotting a power consumption W with respect to a loadvoltage U1;

FIG. 10 is a graph plotting the power consumption W with respect to thecapacitance C of a pulse voltage generator when the load voltage U1 isset at 370 [kV]; and

FIG. 11 is a graph plotting a single pulse energy with respect to alength L between the electrodes.

BEST MODE FOR CARRYING OUT THE INVENTION

The excavation method of the present invention will be describedspecifically.

(i) First, U₀ is determined by use of a sample of a matter to beexcavated: where U₀ is the voltage [kV] applied when the sample iscrushed by the electric discharge from two electrodes having a length of1 cm therebetween placed on the sample of the matter to be excavatedexisting in the discharge liquid.

Micro quartzite (Mineral name: rock crystal) was used as a sample of thematerial to be excavated. The sample was dipped in diesel oil. The twoelectrodes were placed on the sample. The length between the electrodeswas then set to be cm. One of the electrodes was grounded and the otherof the electrodes was loaded in high voltage.

The sample was crushed five times under the same conditions. It is notedthat whatever kinds of samples are used, five to ten times of testingare required. Also, in the case of non-uniform samples, the number ofsamples used should preferably be further increased for testing.

After five times of crushing of samples, a mean value of U₀ =190 [kV]was obtained.

Then, excavation of the rock mass was made using an excavator havingelectrodes of 7 in number (n) and 4 cm in each length L therebetween.The load voltage U1 of 252.8-379.3 [kV] was then given by Eq. (1). Theexcavation was performed while the power consumption W was measured,with the load voltage varied intermittently or continuously within therange of load voltage of 220.0-490.0 [kV] centered about those values.The results are as shown in TABLE 1. At the load voltage U1 of 380 [kV],the power consumption W was minimized.

According to the excavation method of the present invention, theexcavation was made using the optimum load voltage value U1 at which thepower consumption W was minimized. For simplicity, the optimum loadvoltage value U1 should first be selected before the excavating work andthe selected optimum value should then be maintained constant throughoutthe excavating work. For further enhanced efficiency, the step that theload voltage is varied within the range of load voltages including theload voltage value U1 given by Eq. (1), to find an optimum load voltagefor the power consumption W to be minimized should be performed atregular intervals or continuously during the operation of the excavator,so that the excavation can always be made at the optimum load voltagefor the power consumption W to be minimized.

                  TABLE 1                                                         ______________________________________                                        U1   220     250    280  320  360  380  400  440  490                           [kV]                                                                          W No 310 280 240 200 175 180 205 250                                          [J/ dis-                                                                      cm.sup.3 ] charge                                                           ______________________________________                                    

Now, a further detailed description on TABLE 1 will be given here.

The crush of the rock mass was taken place at the rising of the pulse.The crush of the rock mass was dependent on some parameters, the main ofwhich was a voltage across the electrodes. For U1=220 kV, no dischargewas found and thus the rock mass was not crushed. For U1 larger thanthat, electric discharge was observed, but the probability of the rockmass being crushed by the discharge penetration was not more than 100%.

It was found that the voltage rise increased the probability of thedischarge penetration and thus the probability of the crush. However, inmany cases, the excavation in the condition of the energy being mostconsumed does not always maximize the probability of the dischargepenetration. Increase in dielectric breakdown and reduction inreliability of a pulse voltage generator can be cited as the reasonstherefor. This is why the maximum efficiency is achieved at close to themaximum U1 in the instructed Eq. (1), i.e., 379.3 [kV].

In fact, the energy consumption for the crush of the rock mass wasminimized at 380 [kV], as shown in TABLE 1. Under this condition, thebreakdown voltage was thought to achieve the maximum efficiency. A mainreason why the power consumption increased for U1>380 kV is the re-crushof the discharge mud.

(ii) Then, with the load voltage taken as 380 [kV] and the lengthbetween the electrodes taken as 4 cm and also with single pulse energyvaried continuously or intermittently in the range of 250 to 1,550 [J],the excavation of sandstone was performed while the power consumption Wwas measured. The single pulse energy was varied by varying capacitanceof the pulse voltage generator.

It was found that the single pulse energy given by Eq. (2) of W₀ ≧90L¹.6was included in the range of the single pulse energy of 250 to 1,550[J]. The results are as shown in TABLE 2. The power consumption W wasminimized when the single pulse energy W₀ was in the range of 880 to1,100 [J].

According to this excavation method of the present invention, theexcavation is performed by using the optimum single pulse energy valueW₀ at which the power consumption W is minimized. For simplicity, theoptimum single pulse energy value W₀ should first be selected before theexcavating work and the selected optimum value should then be maintainedconstant throughout the excavating work. For further enhancedefficiency, the step that the single pulse energy is varied within therange of single pulse energies including the single pulse energy W₀given by Eq. (2), to find an optimum single pulse energy for the powerconsumption W to be minimized should be performed at regular intervalsor continuously during the operation of the excavator, so that theexcavation can always be made at the optimum single pulse energy for thepower consumption W to be minimized.

                  TABLE 2                                                         ______________________________________                                        W.sub.o [J]                                                                          250     400    650   880  1100  1400 1550                                W [J/Sec.] 300 230 190 180  180  250  280                                   ______________________________________                                    

Now, a further detailed description on TABLE 2 will be given here.

An excavation condition as properly set can allow the excavationefficiency to increase and also allow the necessary energy for the crushto reduce. This will clearly be seen from TABLE 2. The increase of thesingle pulse energy W₀ caused reduction of the power consumption W.Further, when the single pulse energy W₀ was increased, the powerconsumption W was increased due to the re-crush of the discharge mud.The power consumption can be reduced by 2/3 or less by determining themaximum efficiency of the discharge energy from the pulse power source.

(iii) Then, with the single pulse energy taken as 850 [J]; the lengthbetween the electrodes taken as 4 cm; the diameter of the bore top takenas 110 [mm]; and the pulse frequency f taken as 1 to 9, the quantity Qof discharge liquid was calculated. The reason why the pulse frequency fwas taken as 1 to 9 is that while the rate of excavation is directlyproportional to the pulse frequency f in the range of 1 to 9 per second,reduction of the excavation efficiency is caused when the frequency isincreased further. With the quantity of discharge liquid variedcontinuously or intermittently within the range of the quantity ofdischarge liquid including the value given by Eq. (3) above, theexcavation of rock crystal was performed while the then powerconsumption W was measured. As a result, the quantity Q of dischargeliquid of 450 [liter/min.] was found to be optimum.

In this excavation method of the present invention, the excavation isperformed by using the optimum quantity Q of discharge liquid for thepower consumption W to be minimized. For simplicity, an optimum quantityQ of discharge liquid should first be selected before the excavatingwork and the selected optimum value should then be maintained constantthroughout the excavating work. For further enhanced efficiency, thestep that the quantity of discharge liquid is varied within the range ofquantities of discharge liquid including the quantity Q of dischargeliquid given by Eq. (3), to find an optimum quantity of discharge liquidfor the power consumption W to be minimized should be performed atregular intervals or continuously during the operation of the excavator,so that the excavation can always be made at the optimum quantity ofdischarge liquid for the power consumption W to be minimized.

Referring now to FIGS. 1 to 8, the excavator of the present inventionwill be described.

(Structure of Excavator)

In FIG. 1, an excavator 1 is composed of a high-voltage pulse generator2, bore portions 9, 10, 11 and 12, discharge liquid circulating system3, 4, 5a and 5b, optimum condition setting devices 13, 14, 15 and 16,and lifting devices 6, 7 and 8.

The bore portions include a high-voltage input portion 9, a bore pipe11, a guide 10, arranged around the bore pipe 11, for guiding the borepipe 11 into underground, and a bore top 12 provided at the tip end ofthe bore pipe 11.

The bore pipe 11 is composed of a high-voltage pipe 20 and a ground pipe21 arranged concentrically outside of the high-voltage pipe 20 and is sostructured as to be slidable vertically within the guide 10. Thehigh-voltage pipe 20 and the ground pipe 21 are partitioned byintermediate insulators 22a, 22b. The inside of the ground pipe 21 andthe outside of the high-voltage pipe 20 are plated with a non-magneticand high conductive material. The non-magnetic and high conductivematerials which may be used include duralumin, copper, brass andaluminum. This plating is given for the purpose of suppressing aphenomenon that with an increase in depth of a hole to be excavated,pulse rise time increases and a voltage amplitude decreases. The orderof not more than 0.1 mm is an enough thickness for the plating.

The guide 10 and the ground pipe 21 are connected with each otherthrough a sliding contact point 23 so that the ground pipe 21 can slidevertically within the guide 10. This is because a discharge circuit ofthe high-voltage pulse generator 2 and the bore portion is notnecessarily required to be grounded, but the guide 10 and ground pipe 21are required to be connected to each other.

The discharge between a ground electrode 17 and a high-voltage electrode18 may act to the discharge liquid as well, so that when a voltageexists between the guide 10 and the ground pipe 21, the breakdown of airgap may sometimes occur. In order to protect the discharge circuit ofthe high-voltage pulse generator 2 and the bore portion from thebreakdown of air gap, the guide 10 and the ground pipe 12 are thusrequired to be connected to each other through the sliding contact point23.

The bore top 12 is composed of the ground electrode 17 and thehigh-voltage electrode 18. The ground electrode 17 and the high-voltageelectrode 18 are partitioned by a bore top insulator 19. The number ofground electrode 17 and high-voltage electrode 18 is not limited to theonly one for each, as discussed later.

The discharge liquid circulating system includes a discharge liquidreservoir 3, a discharge liquid pump 4 and discharge liquid pipes 5a,5b. The discharge liquid circulating system allows the discharge liquidto circulate, passing from the discharge liquid reservoir 3 through thepump 4 and the discharge liquid pipes 5a, 5b to the bore portion andreturning therefrom through the gap between the outside of the groundpipe 21 and the excavating hole and through the guide 10 to thedischarge liquid reservoir 3.

An optimum condition setting device includes a load-voltage-and-othersadjusting device 13 for adjusting the load voltage, the single pulseenergy or equivalent; a power consumption measuring device 14 such as apulse current transformer; a discharge liquid control device 15; and anoptimum-condition-setting control device 16 for setting optimumcondition. The optimum-condition-setting control device 16 is connectedwith the load-voltage-and-others adjusting device 13, the powerconsumption measuring device 14, and the discharge liquid control device15. This optimum-condition-setting control device 16 operates tooptimize the excavation conditions at regular intervals or continuouslyso that the power consumption can be minimized. The parameters foroptimization of the excavation conditions include a load voltagerequired for the crush of rock mass, a single pulse energy and aquantity of discharge liquid.

The discharge liquid control device 15 is assembled in between thedischarge liquid pipes 5a, 5b and controls the parameters of thedischarge liquid at regular intervals or continuously in accordance withthe optimum-condition-setting control device 16. The parameters of thedischarge liquid include properties of fluid (e.g. mechanical propertiesincluding volume conductivity, flow rate and structure) and operatingcircumstances.

The lifting device includes means for lifting the bore pipe 11, such asa winch 6, a pulley 7 and a wire 8.

Now, operation of the thus structured excavator 1 will be described. Thebore portion is set in a bottom of an excavating hole so that the borepipe 11 can be guided into underground by means of the guide 10. Thedischarge liquid is allowed to circulate by the discharge liquidcirculating system, passing from the discharge liquid reservoir 3through the pump 4 and the discharge liquid pipes 5a, 5b to the boreportion and returning therefrom through the gap between the outside ofthe ground pipe 21 and the excavating hole and through the guide 10 tothe discharge liquid reservoir 3. The high-voltage pulse is applied fromthe high-voltage pulse generator 2 to the high-voltage pipe 20 and thehigh-voltage electrodes 18 in the bore top through the high-voltageinput portion 9. The electric discharge is produced between thehigh-voltage electrode 18 and the ground electrode 17. The electricdischarge passes through the rock mass and thereby the rock mass iscrushed. The crushed rock mass is removed from the excavating hole,together with the discharge liquid, by means of the discharge liquidcirculating system.

Then, the lifting device lowers the bore portion downward to reset it ina newly formed bottom of the excavating hole. The bore pipe 11 is thenguided into underground by the guide 10, while it is slid downward alongan inner wall of the guide 10. The operations above are repeated for theexcavating work to proceed.

During the operations, the load voltage, the single pulse energy and thequantity of discharge liquid required for the crush of rock mass aredetermined at regular intervals or continuously by theoptimum-condition-setting control device 16 so that the powerconsumption can be minimized. The properties of fluid (e.g. mechanicalproperties including volume conductivity, flow rate and structure) andthe operating circumstances are determined by the discharge liquidcontrol device under the instruction from the optimum-condition-settingcontrol device.

Thus, according to the excavator 1 of the present invention, since theexcavating conditions required for the crush of rock mass, such as theload voltage, the single pulse energy and the quantity of dischargeliquid, are optimized at regular intervals or continuously by theoptimum-condition-setting control device so that the power consumptioncan be minimized, the excavation can be performed efficiently incompliance with natural conditions.

Also, since the inside of the ground pipe 21 and the outside of thehigh-voltage pipe 20 are plated with a non-magnetic and high conductivematerial, the phenomenon that with an increase in depth of theexcavating hole, pulse rise time increases and a voltage amplitudedecreases can be suppressed significantly. This can provide the resultthat the pulse conditions need not be changed so often, thus enablingthe stable operation of the excavator.

Further, since the guide 10 and the ground pipe 21 are connected witheach other through the sliding contact point 23 at the outlet side ofthe discharge liquid, a possible breakdown of air gap resulting from theelectric potential difference between the guide 10 and the ground pipe21 can be avoided.

The excavator 1 of the present invention is applicable not only toexcavation of a solid insulating matter such as a rock mass but also toa mining of oil and gas and a civil engineering and construction work.Thus, the matter to be excavated is not limited to the rock mass.

(High-voltage pulse power circuit)

A high-voltage pulse power circuit used in the excavator 1 of thepresent invention will be described with reference to FIG. 2. Thiscircuit is a high-voltage pulse power circuit of inductor capacitor typeusing a semi-conductor rectifier. The high-voltage pulse power circuitincludes a high-voltage capacitor 24, a sphere gap 25, an inductorcapacitor 26 and a semi-conductor rectifier 27 such as a diode. In thediagram, the electrodes 17, 18 of the bore top 12 and the rock mass arealso shown.

The high-voltage pulse power circuit acts in the following manner. Whenthe power source is connected in parallel with the high-voltagecapacitor 24, a voltage is accumulated in the high-voltage capacitor 24.The current i1 which tends to flow toward one electrode 17 of the boretop 12 is then led to the inductor capacitor 26 by means of the diode27, so that the voltage is accumulated and thus increased in theinductor capacitor 26 as well. When the voltage enough to operate thesphere gap 25 is accumulated in the high-voltage capacitor 24, abreakdown occurs in the sphere gap 25 and the high-voltage pulse powercircuit is energized. When the high-voltage pulse power circuit isenergized, the high-voltage current increased in the inductor capacitor26 and the current from the diode 27 flow to the other electrode 18 ofthe bore top 12, as shown by an arrow i2 in the diagram. Then, thedischarge is generated between the electrodes 17, 18 of the bore top 12and thereby the rock mass is crushed. The operations above are repeated.

It is noted that the time interval required for the diode 27 tointerrupt the current to the electrode 17 of the bore top 12 so that thevoltage enough to operate the sphere gap 25 can be accumulated in thehigh-voltage capacitor 24 is as little as nanosecond. The voltage in theinductor capacitor 26 is increased within this little time interval.This increase of voltage can be increased by 3 to 3.5 times as high asthe load voltage to the high-voltage capacitor 24 by setting theconditions such as impedance adequately.

The high-voltage pulse power circuit thus constructed by combination ofthe inductor capacitor 26 and the diode 27 provides the followingadvantages, as compared with a conventional type of excavator in whichthe diode 27 is not used in the power circuit. The form and weight canbe cut in half, thus enabling movement of the excavator; and the numberof capacitors and sphere gaps is reduced and also the capacitor voltagecan be lowered, thus increasing the life of the high-voltage pulse powercircuit.

Also, the use of the inductor capacitor and the solid-state rectifierenables the pulse power source to be further reduced in form and size,as compared with the conventional type one, thus enabling those to bepositioned in the bore, particularly, in the vicinity of the bore top.Accordingly, the pulse power circuit of inductor capacitor type can bedipped in the bore top in the excavating hole. This is an important partwhen a deep hole is excavated, particularly when a discharge liquid highin electric conductivity, such as water, is used.

The single pulse energy can be varied by varying the capacitance.

(High-voltage input portion)

Referring now to FIGS. 3 and 4, a preferable mounting position andstructure of the high-voltage input portion 9 of the excavator of thepresent invention will be described.

In FIG. 3, a support 27, the winch 6 and the wire 8 form the liftingdevice. In the excavator of the present invention, the high-voltageinput portion 9 is arranged inclined from the axis of the bore pipe anat a predetermined angle α. It is preferable that the predeterminedangle α is in the range of 30°<α<150° with respect to the axis of thebore pipe 11.

The wire 8 of the lifting device is connected to the bore pipe can Thebore pipe 11 is guided into underground by means of the guide 10, so asto move up and down along the guide 10.

This inclined arrangement of the high-voltage input portion 9 canfacilitate a connection of the wire 8 of the lifting device to the borepipe 11, as compared with the embodiment of the high-voltage inputportion 9 arranged coaxially on the bore pipe 11. This can facilitatethe setting and lifting of the bore pipe 11 in and from the excavatinghole by means of the lifting device without paying particular attentionto a contact between the high-voltage input portion 9 and the wire 8.Further, this enables the bore pipe 11 to be moved in the excavatinghole even by means of the lifting device.

With reference to FIG. 4, the structure of the high-voltage inputportion 9 will be described. In FIG. 4, reference numeral 29 designatesan insulating part. The insulating part 29 has an inverted cup-like formhaving a through hole 29b at the bottom. The cup-like formed insulatingpart 29 is provided with a flange 29c around an opening thereof. Theflange 29c is connected to a flange 21a of the ground pipe 21 of thebore pipe.

A high-voltage wire 30 is housed in the insulating part 29, with itsupper end portion inserted and fixed in the through hole 29b in such amanner as to project a little from the through hole 29b. A current cable31 is connected to the upper end 30b of the high-voltage wire 30, andthe high-voltage pipe 20 of the bore pipe is connected to the lower endof the same.

The insulating part 29 is coated with a semi-conductive material byapplying the semi-conductive material on at least an outer surface 29athereof. The surface 29a of the insulating part 29 coated with thesemi-conductive material and the ground pipe 21 of the bore pipe have anelectric contact point 29e at their flanges 21a, 29c. Similarly, thehigh-voltage wire 30 is also coated with the semi-conductive material byapplying the semi-conductive material on a surface 30a thereof. Thesurface 30a of the high-voltage wire 30 coated with the semi-conductivematerial and the surface 29a of the insulating part 29 coated with thesemi-conductive material have the electric contact point 29d at a fixingportion of the insulating part 29 to the high-voltage wire 30. Thesemi-conductive materials which may be used include a mixture of solventsuch as polyethylene and graphite and a composite material.

The high-voltage input test was performed by use of the high-voltageinput portion 9 constructed above. The insulating part 29 and the highvoltage wire 30 of high-density polyethylene were used. The high-voltagewire 30 had a diameter of 4 mm and a height of 220 mm. The current cable31 had a diameter of 15 mm. The flange 29c of the insulating part 29 hada diameter of 160 mm.

A resistance between the contact points 29e and 29d on thesemi-conductivity material coated surface 29a was 1.2 kΩ, and aresistance between the upper end 30b of the high-voltage wire 30 whichis the connecting surface with the current cable 31 and the contactpoint 29d was also 1.2 kΩ. This means that the total electric resistanceof the semi-conductive material coated surfaces 30a and 30b was 2.4 kΩ.

Then, a test voltage was applied between the high-voltage pulsegenerator and the current cable, and the test voltage was increasedstepwise to 350 kV-880 kV. The time intervals at which the voltagepulses were applied were set at 1.5 microsecond. It was then found thatthe high-voltage input portion 9 coated with the semi-conductivematerial withstood 2.7 times more load voltage, as compared with the onecoated with no semi-conductive material.

(Discharge mud collecting device)

Referring now to FIGS. 5(a) and 5(b), the discharge mud collectingdevice forming the discharge liquid circulating system will bedescribed. FIGS. 5(a) and 5(b) are a showing of the discharge mudcollecting device mounted to the bore pipe and a sectional view of thesame, respectively. In the diagrams, R designates the diameter of thebore top.

The discharge mud collecting device has two discharge liquid feedingpassageways 32a, 32b and two collecting passageways 33a, 33b for thedischarge mud to be pumped together with the discharge liquid, as shownin FIG. 5(b).

The two discharge liquid feeding passageways 32a, 32b are spaces of asegment section defined by an inner pipe 34 arranged concentrically withthe ground pipe 21 of the bore pipe; two outer walls 35 of a circularlyarc section located at a radially outer side of the inner pipe 34; andfour partition walls 36 projecting radially outwardly from the innerpipe 34 to connect the two outer walls 35 with the inner pipe 34.

The two collecting passageways 33a, 33b are grooves defined between thetwo discharge liquid feeding passageways 32a, 32b. Thus, the dischargemud collecting device is so designed as to form the collectingpassageways, without using any pipes intended for collecting use.

As shown in FIG. 5(a), a plurality of valves 37 of elastic material suchas rubbers formed on the inner pipe 34 are vertically arranged in thetwo collecting passageways 33a, 33b.

Now, operation of the thus structured discharge mud collecting devicewill be described. After the matter to be excavated, such as a rockmass, is crushed, the fragments of the excavated matter such as the rockmass are mixed in the discharge liquid fed from the two discharge liquidfeeding passageways 32a, 32b, resulting in the discharge mud. Thedischarge mud is pumped up through the two collecting passageways 33a,33b by a pump located on the ground.

When the speed allowing the discharge mud to be pumped up is sufficient,even a large fragment is pumped up passing through the elastic valves37. On the other hand, even when the speed allowing the discharge mud tobe pumped up decreases suddenly or comes to be insufficient, thedischarge mud is dropped onto the elastic valves 37 and thus the backflow is avoided. Further, since the two collecting passageways 33a, 33bare grooves defined between the two discharge liquid feeding passageways32a, 32b, not any pipes intended for collecting use, large fragmentsdropped on the elastic valves 37 can be crushed or pulverized by a knowntechnique.

This can eliminate a possible fear, involved in the known discharge mudcollecting device having a collecting passageway formed by a pipe orequivalent, that the collecting passageway may be blocked by largefragments, to cause damage of the collecting pipe and others, which mayin turn cause damage of the discharge mud collecting device itself.

Where the length between the electrodes is set to be not less than 100mm as a property of the electric pulse excavation, there is apossibility of the discharge mud being crushed into large fragments.However, the discharge mud collecting device discussed above preventsthe pipes or equivalent for the collecting passageways from beingdestroyed even when the speed at which the discharge mud is pumped up isdecreased suddenly or the collecting passageways are blocked by thelarge fragments.

(Mounting structure of the bore top)

FIG. 6 is a schematic diagram of a mounting structure of the bore top 12to the bore pipe 11 In the diagram, R designates a diameter of the boretop.

The bore top 12 is threadedly fixed to the ground pipe 21 of the borepipe 11 at the tip end. A horizontally extending aperture 38 having apredetermined length is formed in a thread portion of the bore top 12.Two detents 39a, 39b are provided in the horizontally extending aperture38.

The length between the two detents 39a, 39b is rendered shorter than alength of the horizontally extending aperture 38, in order to allow thebore top 12 to rotate around its axis. The length which allows the boretop 12 to rotate around its axis is determined by the difference betweenthe length of the horizontally extending aperture 38 and the lengthbetween the two detents 39a, 39b. The difference should preferably bethe length between the electrodes and above.

The reason for the provision of the horizontally extending aperture 38is as follows.

In the process of the excavation, the bore top 12 is subjected to ashock wave and an impact from the discharge mud. As a result of this,the bore top 12 is susceptible to bending and may sometimes be twisted.However, since the bore top 12 is provided with the horizontallyextending aperture 38 which can allow the bore top to rotate around itsaxis, it is prevented from being twisted. Then, the bore top 12 is keptat its specified position by the bottom. This produces an enhancedefficiency of the excavation of the rock mass. The efficiency wasimproved by 20%, as compared with the bore top having no horizontallyextending aperture 38.

(Number of electrodes of the bore top and Movement of the bore toparound a center axis of a hole to be excavated)

It is one of the important factors for reduction of excavation costswhat kind of discharge liquid should be used. If water is used as adischarge liquid low in cost, for example, since water has high electricconductivity, a considerable amount of leakage current is producedbetween the electrodes. In view of this, reduction of the number ofelectrodes is significant. However, excavation of a hole of a largediameter had a limitation in reduction of the number of electrodes.

Shown in FIG. 7 is the electrode structure of the bore top which iscapable of reducing the number of electrodes to a minimum while having acapability of excavating a hole of a large diameter.

The bore top is composed of two high-voltage electrodes 18 and twoground electrodes 17. The electrodes are positioned on points ofintersection of grid. In this embodiment in which the bore top has fourelectrodes, the electrodes are positioned at the vertexes of a square.Thus, the electrode arrangement is rectangular, so that the bore toptakes the maximum diameter R.

The bore top thus structured is movable around an axis O of theexcavating hole. The movement of the bore top can be effected by a flowof discharge liquid or an electric discharge energy. Alternatively,modification may be made such as, for example, making the arm 28a at theend of the support 28 of the lifting device shown in FIG. 3 extendableand contractable horizontally as well as pivotable around the support 28at 90° so that the arm can be moved together with the bore pipe 11 toreliably move the bore top to a specified position.

Operation of the bore top thus structured will now be described. In thebore top having the structure discussed above, the electric dischargeoccurs in the parts I enclosed by two-dotted lines and, as a result, thecrush occurs in part of the rock mass protruded in the part II enclosedin the parts I, so that the rock mass is crushed with efficiently. Then,the bore top is moved in the direction of an arrow around the axis O ofa hole 40 to be excavated. The excavating work is started again in theplace to which the bore top is moved and simultaneously a boring work isperformed in the place from which the bore top is moved. Thus, with thebore top moved around the axis O of the hole to be excavated, holeshaving different diameters can be excavated.

The prior art usually requires the number of electrodes by several timesfor the excavation of the hole 40 of such a large diameter, andaccordingly the leak current is increased by several times and theefficiency is reduced significantly.

(End structure of the electrode)

It is hard to make an excavation of a matter to be excavated having acore of a large diameter. The end structure of the electrode suitablefor such an excavation of the matter to be excavated having a core of alarge diameter is shown in FIG. B. The high-voltage electrode 18 and theground electrode 17 are bent at their ends so that one can extend towardthe other with respect to each other. The bending angle shouldpreferably be not more than 90°.

The electrodes having this end structure crushed a central core having adiameter of 600 mm at 100 pulses in total. The excavation efficiency wasimproved by 30e by this method.

(Optimization of excavation conditions)

Next, it is described specifically how the excavation conditions wereoptimized in the excavator 1 of FIG. 1. The length between theelectrodes was set at 4 cm.

In this excavation method, the single pulse energy for minimizing thepower consumption W must be determined with respect to the load voltageU1 for minimizing the power consumption W. This also means thedetermination of an optimum value of a capacitance C of the pulsevoltage generator for minimizing the power consumption W with respect tothe load voltage U1 for minimizing the power consumption W, because thesingle pulse energy is varied by varying the capacitance of the pulsevoltage generator.

FIG. 9 is a graph plotting the electric power consumption W with respectto the load voltage U1. Possible values of the load voltage U1 at whichthe power consumption W may be minimized were estimated by Eq. (1) whichis one of the excavation methods of the present invention. Then, withthe load voltage varied intermittently in the range of load voltages of250-500.0 [kV] including those estimated values, the power consumption Wwas measured. The power consumption W was reduced to the minimum at theload voltage U1 of 370-390 [kV]. Then, the load voltage U1 was set at370 [kV].

FIG. 10 is a graph plotting the power consumption W with respect to thecapacitance C of the pulse voltage generator where the load voltage U1is set at 370 [kV]. Possible values of the single pulse energy W₀ atwhich the power consumption W may be minimized were estimated by Eq. (2)which is one of the excavation methods of the present invention. Then,with the capacitance C of the pulse voltage generator, at which thosesingle pulse energy W₀ is produced, varied intermittently in the rangeof the capacitance C centered near those estimated values, the powerconsumption W was measured. The power consumption W was reduced to theminimum at the capacitance C of 0.014 [μF]. Thus, when the lengthbetween the electrodes was L=4 [cm], the optimum conditions of the loadvoltage being U1=370 [kV] and the capacitance C of the pulse voltagegenerator being C=0.014 [μF] were found.

Then, when the load voltage U1 was set at 70 [kV] and the capacitance Cof the pulse voltage generator was set at 0.014 [μF], the quantity Q ofdischarge liquid which can allow the power consumption W to be minimizedwas determined by Eq. (3) and others to optimize the quantity Q ofdischarge liquid.

It is noted that even when the length between the electrodes is set at avalue other than 4 [cm], the optimum values of the load voltage, thecapacitance C of the pulse voltage generator and the quantity Q ofdischarge liquid can be obtained.

For reference purposes, a graph plotting the single pulse energy withrespect to the length L between the electrodes is shown in FIG. 11.Illustrated therein is a graph of Eq. (3) of W₀ ≧90L¹.6 as an index forthe optimum conditions. The diagonally shaded range in the diagram showsan energy required for the rock mass to be crushed by the electricpulses, as taught by patent to Cretz, Dolzon, et al. (Dated: Sep. 13,1995).

Capabilities of Exploitation in Industry

The present invention is best applicable as the excavation method andthe excavator capable of making excavation efficiently with a minimumpower consumption.

What is claimed is:
 1. An excavation method for crushing a matter to beexcavated, existing in an excavating hole in which a discharge liquid isfed, by means of electric discharge between a plurality of electrodesgenerated by high-voltage pulses, characterized in that at least one ofthe following parameters for excavation efficiency is set to be anoptimum value for minimization of power consumption required forexcavation, before performing the excavation:i) load voltage requiredfor crushing said matter to be excavated; ii) single pulse energy; andiii) quantity of discharge liquid.
 2. An excavation method as set forthin claim 1, wherein an optimum value of said load voltage required forcrushing said matter to be excavated is found by varying said loadvoltage continuously or intermittently within a range of load voltagesincluding a load voltage value U1 given by the following Equation:

    U1=K(1/n-1).sup.0.15 ×U.sub.0 ×L.sup.0.4 [kv]  (1)

Where K: a coefficient, K=1.0-1.5 [1/cm⁰.4 ]; n: the number ofelectrodes; L: a length between the electrodes [cm]; and U₀ : a valueobtained by testing, or a voltage [kv] applied when a sample of thematter to be excavated existing in said discharge liquid is crushed viatwo electrodes having the length of 1 cm therebetween placed on saidsample.
 3. An excavation method as set forth in claim 1, wherein anoptimum value of said single pulse energy is found by varying saidsingle pulse energy continuously or intermittently within a range ofsingle pulse energies including a single pulse energy W₀ given by thefollowing Equation:

    W.sub.0 >90L.sup.1.6 [J]                                   (2)


4. An excavation method as set forth in claim 1, wherein an optimumvalue of said quantity of discharge liquid is found by varying saidquantity of discharge liquid continuously or intermittently within arange of quantity of discharge liquid including quantity Q of dischargeliquid given by the following Equation:

    Q=(0.25-0.5)π×Db.sup.2 /4×f [liter/min.]    (3)

Where Db: a diameter of a bore top [cm]; and f: the number of pulses persecond (frequencies of pulse).
 5. An excavator 1 for crushing a matterto be excavated, existing in an excavating hole in which a dischargeliquid is fed, by means of electric discharge between a plurality ofelectrodes generated by high-voltage pulses, said excavator comprising:ahigh-voltage pulse generator 2; a plurality of electrodes 17, 18, atleast one of which is given a high voltage from said high-voltage pulsegenerator 2; discharge liquid circulating system 3, 4, 5a, 5b; andoptimum condition setting devices 13, 14, 15, 16,said optimum settingdevices 13, 14, 15, 16 being connecting between said high-voltage pulsegenerator 2 and said plurality of electrodes 17, 18, or assembled insaid discharge liquid circulating system 3, 4, 5a, 5b, or connectedbetween said high-voltage pulse generator 2 and said plurality ofelectrodes 17, 18 and assembled in said discharge liquid circulatingsystem 3, 4, 5a, 5b, so that at least one of the following parametersfor excavation efficiency is optimized so that power consumptionrequired for excavation can be minimized: i) load voltage required forcrushing said matter to be excavated; ii) single pulse energy; and iii)quantity of discharge liquid.
 6. An excavator 1 for crushing a matter tobe excavated, existing in an excavating hole in which a discharge liquidis fed, by means of electric discharge between a plurality of electrodesgenerated by high-voltage pulses, said excavator comprising:ahigh-voltage pulse generator 2; a plurality of electrodes 17, 18, atleast one of which is given a high voltage from said high-voltage pulsegenerator 2; and a bore pipe 11, including a high-voltage pipe 20 and aground pipe 21 arranged concentrically outside of said high-voltage pipe20, for conveying said high voltage to said electrodes between saidhigh-voltage pulse generator 2 and said plurality of electrodes 17, 18,said bore pipe being connected to said high-voltage pulse generator 2 atone end portion thereof and connected to said plurality of electrodes atthe other end portion thereof, wherein an inside of said ground pipe 21and an outside of said high-voltage pipe 20 of said bore pipe 11 areplated with a non-magnetic and high conductive material.
 7. An excavator1 for crushing a matter to be excavated, existing in an excavating holein which a discharge liquid is fed, by means of electric dischargebetween a plurality of electrodes generated by high-voltage pulses, saidexcavator comprising:a high-voltage pulse generator 2; a plurality ofelectrodes 17, 18, at least one of which is given a high voltage fromsaid high-voltage pulse generator 2; a bore pipe 11, connected to saidhigh-voltage pulse generator 2 at one end portion thereof and to saidplurality of electrodes at the other end portion thereof, for conveyingsaid high voltage to said electrodes between said high-voltage pulsegenerator 2 and said plurality of electrodes 17, 18; and a guide 10,arranged around said bore pipe 11, for guiding said bore pipe 11 intounderground, said bore pipe 11 and said guide 10 being connected witheach other through a sliding contact point 23 so that said bore pipe 11can slide vertically within said guide
 10. 8. An excavator 1 forcrushing a matter to be excavated, existing in an excavating hole inwhich a discharge liquid is fed, by means of electric discharge betweena plurality of electrodes generated by high-voltage pulses, saidexcavator comprising:a high-voltage pulse generator 2; and a pluralityof electrodes 17, 18 at least one of which is given a high voltage fromsaid high-voltage pulse generator 2, wherein said high-voltageelectrodes 18 and other electrodes 17 to which said high voltage isinput are bent at their ends so that one can extend toward the otherwith respect to each other.
 9. An excavator 1 for crushing a matter tobe excavated, existing in an excavating hole in which a discharge liquidis fed, by means of electric discharge between a plurality of electrodesgenerated by high-voltage pulses, said excavator comprising:ahigh-voltage pulse generator 2; a plurality of electrodes 17, 18, atleast one of which is given a high voltage from said high-voltage pulsegenerator 2; a vertically extending bore pipe 11, connected to saidhigh-voltage pulse generator 2 at one end portion thereof and to saidplurality of electrodes at the other end portion thereof, for conveyingsaid high voltage to the electrodes between said high-voltage pulsegenerator 2 and said plurality of electrodes 17, 18; and a liftingdevice having a lifting means for moving said bore pipe 11 up and down,wherein a high-voltage input portion 9 for inputting a high voltage tosaid bore pipe 11 is arranged at one end portion of said bore pipe 11inclined from an axis of said bore pipe 11 at a predetermined angle α.10. An excavator 1 for crushing a matter to be excavated, existing in anexcavating hole in which a discharge liquid is fed, by means of electricdischarge between a plurality of electrodes generated by high-voltagepulses, said excavator comprising:a high-voltage pulse generator 2; aplurality of electrodes 17, 18, at least one of which is given a highvoltage from said high-voltage pulse generator 2; and a bore pipe 11,including a high-voltage pipe 20 and a ground pipe 21 arrangedconcentrically outside of said high-voltage pipe 20, for conveying saidhigh voltage to said electrodes between said high-voltage pulsegenerator 2 and said plurality of electrodes 17, 18, said bore pipebeing connected to said high-voltage pulse generator 2 at one endportion thereof and to said plurality of electrodes at the other endportion thereof, wherein an outer surface of a high-voltage inputportion 9 for inputting said high voltage to said bore pipe 11 is coatedwith a semi-conductive material and is electrically connected with saidground pipe
 21. 11. An excavator 1 for crushing a matter to beexcavated, existing in an excavating hole in which a discharge liquid isfed, by means of electric discharge between a plurality of electrodesgenerated by high-voltage pulses, said excavator comprising:ahigh-voltage pulse generator 2; a plurality of electrodes 17, 18, atleast one of which is given a high voltage from said high-voltage pulsegenerator 2; a bore pipe 11, connected to said high-voltage pulsegenerator 2 at one end portion thereof and to said plurality ofelectrodes at the other end portion thereof, for conveying said highvoltage to said electrodes between said high-voltage pulse generator 2and said plurality of electrodes 17, 18; and a discharge mud collectingdevice fixed to said bore pipe 11, wherein said discharge mud collectingdevice has discharge liquid feeding passageways 32a, 32b which areformed by pipes having a segment section and arranged concentricallywith said bore pipe 11; and collecting passageways 33a, 33b which areformed by grooves defined between said discharge liquid feedingpassageways 32a, 32b and in which a plurality of elastic valves 37 arearranged along a direction of said discharge mud being collected.
 12. Anexcavator 1 for crushing a matter to be excavated, existing in anexcavating hole in which a discharge liquid is fed, by means of electricdischarge between a plurality of electrodes generated by high-voltagepulses, said excavator comprising:a high-voltage pulse generator 2; abore top 12 having a plurality of electrodes 17, 18 at least one ofwhich is given a high voltage from said high-voltage pulse generator 2;and a bore pipe 11, having one end portion to which said high-voltagepulse generator 2 is connected and the other end portion to which saidbore top 12 is threadedly fixed, for conveying said high voltage to saidelectrodes between said high-voltage pulse generator 2 and said bore top12, wherein a threaded portion of the bore top 12 is provided with ahorizontally extending aperture 38 having a predetermined length and twodetents 39a, 39b arranged in said horizontally extending aperture 38,and wherein a length between said two detents 39a, 39b is renderedshorter than said predetermined length of the horizontally extendingaperture 38, in order to allow said bore top 12 to rotate around itsaxis.
 13. An excavator 1 for crushing a matter to be excavated, existingin an excavating hole in which a discharge liquid is fed, by means ofelectric discharge between a plurality of electrodes generated byhigh-voltage pulses, said excavator comprising:a high-voltage pulsegenerator 2; a bore top 12 having a plurality of electrodes 17, 18 atleast one of which is given a high voltage from said high-voltage pulsegenerator 2; and a bore pipe 11, having one end portion to which saidhigh-voltage pulse generator 2 is connected and the other end portion towhich said bore top 12 is connected, for conveying said high voltage tosaid electrodes between said high-voltage pulse generator 2 and saidbore top 12, wherein said bore top 12 is provided with said electrodesat points of intersection of grid and is movable in said excavatinghole.